We are studying the interaction of the thymidine analogue, Bromodeoxyuridine (BrdU) and the mechanisms which regulate the redundancy of the ribosomal RNA cistrons of Drosophila. In progress are experiments designed to determine if BrdU affects sequences other than the rDNAs, if BrdU must be incorporated into DNA to have its effects, and if the drug specifically affects regulatory systems known to establish and maintain rDNA redundancy.